Drilling systems may use a downhole motor powered by drilling fluid pumped from the surface to rotate a drill bit. Most commonly, a positive displacement motor of the Moineau type, which utilizes uses a spiraling rotor that is driven by fluid pressure passing between the rotor and stator, is employed. Other motor types, however, including turbine motors, may be used as appropriate. The downhole motor and bit may be part of a bottom hole assembly supported from a drill string that extends to the well surface.
The cost to drill a well may be significantly affected by the effective rate of penetration (“ROP”) while drilling. As well depth increases, formation rock strength may increase, and the increasing rock strength may result in decreased rate of penetration. It may be desirable, therefore, to increase rock cutting efficiency and/or to reduce the required rock cutting force. Reduced cutting force may result in lower drill bit wear and breakage, less frequently encountered stick-slip conditions, lower probability of shearing the drilling string, and a concomitant greater effective rate of penetration.